Birch Leafminers
Birch leafminers have recently become among the most common insects affecting Birch trees (Betula spp.) They form blotch mines as the larvae feed inside the leaves. Areas inside the leaves are consumed affecting the leaves ability to produce food. Birch leafminers are sawflies, which are closely related to bees and wasps. Yearly browning of birch leaves are noticed in mid July and August, but the leafminers have been feeding inside the leaf tissue since early spring.
Leafminers overwinter in the soil as prepupae. Adults emerge in May to late June to early July, depending on temperature and humidity. Oviposition (egg-laying) was observed to peak during the last week of June. Adult birch leafminers are small (about 1/8 to 1/4 inch long), black and fly like. Females deposit their eggs singly in slits cut in the central areas of young leaves, usually near the tips of branches. More than one female may lay eggs in a leaf.
The eggs hatch into legless, worm-like larvae. These immature larvae feed individually between the leaf surfaces, creating blotchy kidney shaped mines. The immature leafminers feed for several weeks, then drop to the ground where they enter the soil layer to develop into pupae. They pupate and remain there until the following spring. After overwintering as prepupae in the soil below the tree, the adults emerge just as the birch trees are leafed out. Adults are almost always all females.
The areas of leaves that are consumed by the amber marked birch leafminer larva turn brown. Because people often do not see the early signs of birch leafminer feeding, it often appears the tree has suddenly dried up or become diseased. This browning is caused by the outer layers of the leaf drying out after the leaf miner larva has consumed the green tissue between the outer layers of the leaf. Early mines appear as light green or whitish discolorations on the leaves. Larvae sometimes can be seen easily when leaves are held up to sunlight, especially as the mines and larvae grow larger. Feeding over several weeks causes the blemish to take on a blister-like appearance. A single leaf can contain as many as 40 larvae whose mines may merge to destroy the total photosynthetic area of the leaf. Heavy infestations of leafminer larvae can seriously affect a tree's photosynthetic capacity. Repeated attacks will generally cause stress which may induce susceptibility of the tree to other injurious agents.
The most common species responsible for defoliation and browning of birch trees in Northern forests in the United States and Canada is the Amber Marked Leafminer, Profenusa thomsoni, which were accidentally introduced from Europe to North America early in this century. The Birch Leafminer, Fenusa pusilla, was long thought to be the species responsible, but in 2002 the U.S. Forest Service entomologists in Alaska determined that the Amber Marked Leafminer, (Profenusa thompsoni) was the only species present in those samples taken from the Anchorage area.
Presently there is no commercially available biological control agent to control Amber marked birch leafminers, however Canadian trees in the Edmonton area have been successfully controlled with releases of a parasitic wasp, Lathrolestes luteolator. The possibility exists that the parasitoid will provide some control in Alaska when supplies become available. Populations of the tiny parasitoid wasp selectively attack the most damaging birch leafmining pest (Profenusa thomsoni) have developed and drastically reduced the problem in the Edmonton area of Canada.
Following trials in 1995 that supported a dramatic reduction in birch leafminer damage by the first parasitoid, the City of Edmonton discontinued pesticide treatments to almost 3500 city birch trees in 1996 and 1997. These trees continue to show very little leafminer damage without any treatment.
Spinosad should be used to control birch leafminers prior to extensive damage. Spinosad is a new chemical class of insecticides derived from a soil dwelling bacterium discovered in 1982. It is considered practically non-toxic to humans, pets, and beneficial insects. Unlike other insecticides, Spinosad will not harm beneficial insects including the Amber Marked Leafminer parasite. Call IPM of Alaska for more information about Spinosad.
Horticultural oil applications applied at the right time may help kill eggs or tiny larvae within the leaf tissue. Oil applications should be made as soon as adults have emerged in the spring and egg laying has occurred and should continue weekly until mid June. Pesticides made with botanical plant oils may be especially useful to prevent egg laying. Neem oil acts as a repellent and may interfere with the egg laying activity of female leaf miners.
Systemic Insecticides are chemical pesticides that are absorbed into the tissues of plants. These pesticides make the entire plant, or parts of the plant, poisonous to insects that feed on the plant tissue. Most systemics are very toxic to people and pets. We do not recommend the use of the extremely toxic systemic insecticide Metasystox® which is applied to the soil. Soil applied systemic injections have the great potential for ground water contamination and may contact children and pets, therefore their use is never justified in Alaska. If a systemic insecticide is necessary, use acephate (Acecap97®), abermectin (Mauget System Abacide®), or imidacloprid, such as the Mauget System Imicide® or Wedgle System Pointer®) applied as a trunk injection. Cygon® (dimethoate) is no longer available, and should never be used due to its extreme toxicity. Apply either acephate, abamectin, or imadacloprid as a systemic trunk injection prior to the first week of June. Trunk injections are confined to the trees cambium layer, where it is carried to the leaf tissue by the movement of the trees sap. Do not use acephate (Orthene®) or imidacloprid (Merit®) as a foliar application due to their impact on beneficial insects.
Roots of birch trees need a cool, moist, shady location. Proper site selection is crucial for a long, healthy existence.
Fertilizing is best done in early spring at the onset of the growing season. Lawn fertilizer applications around the tree may be sufficient, however additional applications of a high phosphorous fertilizer such as 8-32-16 or super phosphate 0-50-0 in the fall will aid in root development. If you use natural fertilizer, adding additional mycorrhizal fungi, a symbiotic mycelium found in fungal based compost tea, to the soil around birch will help the trees utilize nutrients and increase the health of the soil
Prune any dead wood and remove the smaller of any branches that rub one another. Birch tree pruning is best done after the leaves are fully developed (late June to Mid July).
During the growing season, provide water during prolonged drought conditions. Thoroughly soak the area under and around the tree at least once a week with at least one inch of water.
To reduce the risk of mechanical damage from lawn mowers, weed eaters etc., remove any sod from an area immediately surrounding the tree trunk. In this area it's best to use a weed barrier or mat under an organic mulch to control weed development.
If your tree was severely attacked last year, clean up the leaf litter beneath the tree and vigorously rake and/or burn (if allowed) beneath the tree to destroy overwintering prepupae. Simply removing the leaf litter without disturbing the top 1-2 inches of soil beneath the trees will not destroy the overwintering leafminers.
IPM of Alaska. "The Problem: Birch leafminers." [www.ipmofalaska.com/files/leafminers.html] Natural Resources Forestry Canada, Northern Forestry Centre. Edmonton, Alberta. For. Leafl. 5. Urban Forestry, City of Edmonton, Alberta Canada. Parkland Services Urban Insect and Disease Report. Ed Holsten, Research Entomologist USDA Forest Service: Alaska Region; S&PF, FHP & PNW, personal communication. USDA Forest Service, Amber Marked Birch Leafminer Leaflet R10-TP-114 April 2003.
